1. Field of the Invention
The present invention is directed to a telecommunication switching system for interconnecting different types of telecommunication terminal equipment and, more particularly, to a telecommunication switching system having processing distributed in switch units connected by a bus.
2. Description of the Related Art
Known telecommunication switching systems for locally connected terminal equipment which are commonly used in applications, such as private branch exchanges, are typically one of two types. The first type uses a centralized processing system which may have one or more processors, particularly for redundancy. This centralized processing system performs all switching operations for the entire system and thus must have sufficient capacity to handle a maximum amount of terminal equipment, i.e., telephones, facsimile machines, etc. When used at maximum capacity, the first type of system can be very efficient. However, the ability to expand the system or to use the same type of system at many installations having varying amounts of terminal equipment is often desired. The first type of switching system with its centralized processing system will not be run at maximum efficiency at most installations.
The second type of conventional switching system is designed to provide greater flexibility. Processing is distributed in modules of the terminal equipment. Each module usually uses a processor to control switching and the modules are connected, so that data and voice samples can be interchanged. This design enables the second type of switching system to provide higher efficiency in a larger number of applications than the first type of system, by adding or removing modules. As long as the terminal equipment connected to the switching system is reasonably homogenous, e.g., all telephones of the same type, the second type of system has a relatively simple construction with the same interfaces connected to and operations performed by all of the processors. However, in other environments where there is a mixture of integrated systems digital network (ISDN) terminal equipment and analog terminal equipment, such as analog telephones and radios, different nodes are provided for different types of terminal equipment. The processors have to be programmed differently, despite providing many similar functions, thus increasing the programming effort to create and maintain the switching system, when additional functions are added. For example, if the ISDN terminal equipment which can be connected to the switching system is upgraded or additional types of equipment are added requiring changes to the program in the processors controlling the nodes for ISDN equipment, these changes may require changes to the programs in the processors in the nodes for analog terminal equipment. Systems of the second type often have more than one type of ISDN interface or more than one type of analog interface, each designed to support a specific type of telecommunication terminal equipment. As a result, a new interface board has to be added every time a different type of equipment is connected to the system and maintenance of the software becomes increasingly complicated.